Method and apparatus for stabilization of vessels



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METHOD AND APPARATUS FOR STABILIZATIOH 0F VESSELS 4 Sheets-Sheet 2 Filed Oct. 3l, 1966 INVENTOR. M42/d' S4/W fum.

A, 1910 M. c. Cm. 3,487,8o1

METHOD AND APPARATUS FOR STABILIZATION 0F VESSELS ggw WM5@ 90 1 can/rf@ INVENTOR. MAE/0 (I (AMV BY if@ Jam., 6, 1 970 y M, c. @ALW 3,487,80

METHOD AND APPARATUS FOR STABILIZATION OF VESSELS Filed Oct. 3l, 1966 4 Sheets-Sheet 4 ff/61 5'@ BY #Waff/WY United States Patent O US. Cl. 11d-125 16 Claims ABSTRACT OF THE DISCLGSURE A ship stabilization system having passive tanks at opposite sides of the ship with an interconnecting passage for the flow of liquid between the tanks. The effective cross-sectional area of the interconnecting passage is varied to maintain the natural period of flow of the liquid in excess of the period of roll of the vessel. The crosssectional area is varied in one embodiment by shifting transversely one of the walls defining the passage, and in another embodiment by dividing the passage by a plurality of parallel plates, and closing olf one or more of the passages thus formed. The control may be manual or automatic in response to changes in the period of successive rolls.

This invention relates to the stabilization of vessels and more particularly to the type of stabilization generally referred to in the art as passive tank stabilization.

The usual passive tank stabilizer system includes reservoirs or wing tanks on both sides of the ship, which are connected by a duct at the bottom of the tank thereby forming essentially a modified U-shaped tank. A major shortcoming of prior passive tank stabilizers has been that the periods of the tank and ship may fall into resonance thereby causing an increased roll rather than a decreased roll and, in severe cases, possibly causing the ship to capsize.

Various drawbacks have been found in prior passive tank stabilizers rendering them either relatively ineiiicient in reducing roll, or actually detrimental, under various sea conditions. Such tank stabilizers today generally have been designed for a particular natural period of a ship, but permit a limited period change of the tank stabilizer fordifferent loadings of the ship, and thus only provide efficient stabilization at, or close to, this period. However, a ship at sea has both its natural period, and a range of periods as influenced by the sea and assumed by the ship. For example, a ship may have a natural period of approximately eight and one-half seconds for a particular loading, but the actual periods assumed by the ship for various sea conditions may range on a small vessel from, for example, approximately tive to thirteen seconds. It is therefore desirable to provide eiiicient stabilization of the ship over the expected range of periods to be encountered in service.

Applicant has found that for proper stabilization the natural period of a passive tank stabilizer must be maintained slightly in excess of the period of the vessel. If this is not done, the period assumed by the vessel under various sea conditions may become longer than that of the tank and the two periods fall in resonance resulting in the roll of the veessel bcoming greater than if the tank stabilizer were not used at all. In asmuch asthe period assumed by the ship may vary rapidly, even from roll to roll, it is frequently impossible or impractical to change the natural period of the tank by varying the rice quired for a liquid level change. Furthermore, when the liuid level is reduced, the available corrective moments likewise are reduced, thereby reducing the efficiency of the system. Control valves in the ducts interconnecting the tanks have caused a loss in kinetic energy of the flowing liquid resulting in a consequent reduction of available corrective moment and a substantial reduction in stabilization eliciency. This loss of energy occurs principally because of the amount of turbulence which is created by such valves.

In accordance with the present invention it has been found that to provide proper stabilization it is necessary to substantially continuously vary the natural period of a tank stabilizer as a function of the period of the vessel to prevent possible resonance between the tank and vessel. Resonance occurs when the oscillations of the liquid in the tank lag the roll of the vessel by one hundred and eighty degrees in which case the liquid moves to the low side of the vessel rather than to the high side thereof. Although the maximum amplitude in liquid level difierential occurs when the periods of the tank and vessel are equal, in which case there is a ninety degree phase lag and thus the maximum corrective moments are available, it is relatively easy for the period of the vessel to vary slightly thereby causing the period of the tank to lag the period of the vessel by one hundred and eighty degrees. This will cause an increase in the roll of the ship rather than a decrease. lf the natural period of the tank is maintained slightly in excess of the period of the vessel (the ratio of tank period to vessel period is greater than one), the phase lag between the two is closer to zero degrees which is more desirable, and this also ensures a substantial corrective moment but at a slight loss in amplitude. In order to provide a range of natural periods for the tank without a substantial loss of energy and resulting reduction in available corrective moments, the natural period of the tank is changed by varying substantially the entire effective area of the duct in the tank. In an ideal system, the natural period of the tank may be varied in almost infinite increments over a wide range by providing a large range of area change of the entire duct.

Accordingly, it is an object of the present invention to enable improved stabilization of a vessel to be obtained.

It is an additional object of the invention to provide an improved stabilization system wherein stabilization is achieved over a greater range of periods of a vessel.

A further object of this invention is to provide a passive tank stabilization system wherein the period of a vessel is substantially constantly measured and the natural period of a tank stabilizer is changed `as necessary as a function of the period of the vessel.

An additional object of the invention is to provide a stabilization system wherein the natural period of a tank stabilizer is maintained in excess of the period of a vessel.

Another object of this invention is to provide la passive tank stabilizer in which the natural period thereof is varied substantially continuously as a function of the period of a vessel and maintained in excess of the period of the vessel.

A further object of this invention is to provide a passive tank stabilizer wherein the entire effective area of a duct connecting the reservoirs of the tank is varied as a function of the period of a vessel.

These and other objects and features of this invention will be better understood through a consideration of the following description taken in conjunction with the drawings in which: liquid level therein because of the time and power re- FIGURE 1 is a diagrammatic cross-sectional illustration of the hull of the ship showing a passive tank stabilizer according to the present invention;

FIGURE 2a is ya view similar to FIGURE 1, but illustrating another form of the invention;

FIGURE 2b is a cross-sectional view taken along a line 2b-2b of FIGURE 2a;

FIGURE 3a is a cross-sectional view of a ship hull and shows in more detail another form of a stabilizer according to the invention;

FIGURES 3b and 3c are respective plan and end views of the hull and stabilizer shown in FIGURE 3a;

FIGURE 4 is a block diagram of a control system for controlling the operation of a passive tank stabilizer according to the invention;

FIGURE 5w is a circuit diagram of an exemplary control system according to the invention;

FIGURE 5b is a diagrammatic illustration of an exemplary actuator for a passive tank stabilizer according to the invention;

FIGURE 6 is an illustration of cams employed in the timing portion of a control system shown in FIGURE 5a..

Referring now to the drawings, FIGURE l is a diagrammatic cross-sectional illustration of the hull of a vessel, such as a ship, and a passive tank stabilizer according to the present invention. The hull of the ship is composed of a port and starboard sides 10 and 11 curving down to a bottom 12. The bottom 12 may be the top of the double bottom of the vessel. A bulkhe-ad 13 extends between the sides and bottom to define one end of a tank 14, and the opposite end of the tank (on the plane of the drawing) is covered by another bulkhead (not shown). The tank 14 includes sides or reservoirs 15 and 16 connected by a duct 17. The tank 14 is closed by top members 18 and 19 and a movable member which may be termed a movable block. The block preferably is hollow, and it serves to dene the inner sides of the reservoirs 15 and 16 and the duct 17. The tank 14 may extend to a deck which then provides a top for the tank. The movable block 20 is suitably guided through an opening dened by the top members '18 and 19, and may be moved up or down by a rack 22 and pinion 23 to vary the entire cross-sectional area of the duct 17. It will be appreciated that the block 20 extends from end to end of the tank, and the bottom 24 of the block 20 and bottom 12 define the duct 17. A liquid 26, such as Water or fuel oil, is provided in the tank 14, and an air tube 26 is connected between the sides 15 and 16 to allow undampened motion of the liquid in the tank.

According to the invention, the natural period of the tank is varied to maintain it slightly in excess of the period of the vessel. This is accomplished by sensing the period of the vessel, and moving the block up or down to change the natural period of the tank 14. The period of the vessel may be sensed with a stop watch, or automatically, and the block position may be varied manually or automatically. The manner in which the period of the vessel may be sensed automatically and the position of the block controlled in response thereto will be discussed subsequently.

Since the area of the entire duct 17 is changed by moving the block 20, there is substantially no turbulence of the liquid through the duct 17 thereby preventing the substantial loss of energy which occurs in typical tank stabilizers. It is preferable that the radius of curvature of the sides 10 and 11 be large as practicable in order to prevent an outward force perpendicular to the sides of the tank to eliminate any sudden kick as the liquid 25 moves from side to side.

In the subsequent detailed discussion of the manner in which a passive tank stabilizer may be constructed and operated according to the teachings of the present invention, it will become apparent that the best stabilization is provided by maintaining the period of the tank slightly in excess of the period ofthe ship by continuously varying the pQSitQn. Qi the bleek 2() in @Ssentially infinit@ incr@- 4 ments within the available range of movement. Although a block which is movable in this manner provides good stabilization, a typical block is relatively large and heavy requiring a substantial amount of power to move the block up and down.

An alternative arrangement requiring less power for operation includes a fixed block, a movable gate or gates and vanes Within the duct. An arrangement of this nature is illustrated in FIGURES 2a and 2b in which a fixed block 30 is provided along with movable gates 31 and 32. The sides of the fixed block serve to define the reservoirs 15 and 16, and the bottom of the block defines the maximum height of the duct 17. A plurality of stationary vanes 33 through 35 are mounted in the duct 17 parallel to the upper and lower walls thereof which ar'e defined by the bottom 12 and a bottom 36 of the block 30. The gates 31 and 32 are interconnected by a member 37 which enables the gates to be moved Vup or down together. Both the rear bulkhead 13 and a forward bulkhead 38 are shown in FIGURE 2b.

The vanes 33 through 35, along with the gates 31 and 32 which abut the ends of the vanes when the gates are lowered and close off increments of the duct, enable the natural period of the tank 14 to be changed while still retaining full iluid tiow with minimum turbulence through the remaining open portion of the duct 17. By lowering the gates 31 and 32 in four increments for example, the cross-sectional area of the duct 17 can be varied in four increments from fully open to completely closed. A fewer or greater number of vanes -may be used to provide a different number of increments. The vane and gate arrangement causes a minimum loss of energy of the fluid 25 during stabilization. Additionally, the weight of the gates 31 and 32 is relatively small compared to the weight of a movable block, and hence changes in the natural period of the bank may be more easily accomplished manually or accomplished with a relatively small power source and substantially faster. The gates may be operated through any suitable mechanism, such as a rack and pinion, and manually or by means of one or more motors, such as electric, hydraulic or pneumatic motors.

Another form of the invention is illustrated in FIG- URES 3a through 3c, and these figures illustrate in more detail an embodiment of a tank stabilizer according to the present invention within the hull of a ship. The hull includes port and starboard sides 42 and 43 extending upwardly from a keel 44 and joining a cabin deck beam 45. A double bottom vessel is illustrated, with the top of the double bottom being shown as a flat plate or member 46. The member -46 forms the bottom of a tank 47 having reservoirs or sides 48, 49, and a duct 50. Bulkheads 51 and 52 -form the fore and aft ends of the tank 47 as shown in FIGURES 3b and 3c, and the underside of the cabin deck denes the top of the tank. A xed block, which defines the reservoirs and the maximum duct height, is formed by the bulkheads 51 and 52, side plates 53 and 54, and a bottom plate 55. Bulkhead 51 and plate 57 form the ends (fore and aft) of the duct 50. Various L-shaped and T-shaped stiiieners 58 are shown in FIGURlES 3a through 3c. An air duct 59 is connected between the reservoirs 4S and 49.

A plurality of vanes 62 through 64 are mounted in the duct 50 parallel to the member 46 and the bottom plate 55 of the block. These vanes may be, for example, one-half` inch metal plates, and they divide the duct 50 into four chambers "66 through 69. Instead of a movable block or a pair of gates as illustrated in FIGURES 1 and 2, the tank stabilizer shown in FIGURE 3 includes a single centrally mounted gate 70 which may be lowered through openings 71 in the vanes 62 through 64 and an opening 72 in the plate 55. It will be appreciated that the gate 70 extends from end to end (fore to aft) of the duct 50, and that suitable guides within the duct may be ,provided for guiding the gate as it is moved up and down. The gate 70 is in a water tight enclosure 73 having stuiiing boxes 74 at the top thereof to prevent a liquid in the tank 47 from liowing past the gate 70 into the block. Rods 75 extend through the stutiing boxes 74 and are coupled with the upper end of the gate 70 and with movable dovetailed racks 76. Pinions 77 are coupled by a shaft 78 to a gear box 79. Although the gate 70 may be operated manually, an actuator 80 coupled to the gear box 79 .may be provided to raise and lower the gate 70 through the rack and pinion mechanism. The actuator 80 may take any of various forms such as one or more electric, hydraulic or pneumatic motors.

The single gate conguration shown in FIGURE 3 is simpler than the double gate arrangement shown in FIGURE 2, and since only a single gate is employed, it may be more easily operated manually or the power requirements for the actuator 80 are less. The gate 70 serves the same function as the gates 31 and 32 in FIGURE 2, and may be lowered to reduce the entire effective area of the duct 50 in increments as determined by the spacing of the vanes 62 through 64. As an example of approximate typical dimensions for a ship which is twenty live feet `from side to side at the cabin deck 45 as shown in FIGURE 3a, the duct 50 may be 5.67 feet wide from end to end (fore and aft), the block twelve feet long between plates 53 and 54 thereby giving a duct length of twelve feet. The member 46 is fourteen feet long, and the sides 42 and 43 have a radius of curvature of 8.42 feet. The clear space between the member 46 and the vane 62 is 3 inches the clear space between the vanes 62 and 62 is 1%; inches, the clear space between the vanes 63 and 64 is 23/16 inches, and the clear space between the vane 64 and bottom 57 of the block is 3 inches. With the vanes 62 through 64 each having a thickness of one-half inch, the depth of the duct 50 thus is eleven inches. The spacing of the vanes '62 through 64 is determined by the desired natural periods of the tank 47.

FIGURE 4 illustrates a block diagram of a system for continuously sensing the period of a vessel and for controlling the position of a movable block, or one or more gates, of a tank stabilizer according to the present invention. A period sensor 88 is provided for continuously sensing the period of the ship. The sensor 88 may, in its simplest form, be a pendulum which operates a switch, or may be more elaborate and include a gyroscopic system for sensing the ship period. A timer 89 is connected to the period sensor 88 for measuring the period, and in an exemplary embodiment to be discussed subsequently, may measure the time of each half period of the ship. The time of each period, or half period, is detected by a control circuit 90 which in turn operates the actuator 91 to control the position of the block, gate or gates. In this manner, the period of the ship may be continuously, or substantially so, measured and the resulting information employed for maintaining the natural period of the tank stabilizer at a desired period slightly in excess of the period of the ship. It will be appreciated that the control system illustrated in FIGURE 4 may take any of various forms, such as pneumatic, hydraulic, electromechanical or electronic. In an electronic system, for example, the timer 89 may comprise an electronic counter which in turn operates through logic gates and ampliliers to control the actuator 91. Furthermore, the timer 89 may provide a visual readout which may be observed and used for manual control of the period of the tank.

A relatively simple electromechanical control system for continuously measuring each half period of a ship and providing electrical output signals for operating an actuator is shown in FIGURE 5a, and a diagrammatic illustration of an actuator is shown in FIGURE 5b. A period sensor 88 includes a pendulum 96 coupled with a freely rotatable shaft 97. A collar 98, which may be made of plastic, is mounted on and frictionally engages the shaft 97, but the collar may turn relative to the shaft. A contact arm is mounted on the collar 98 so as to turn with the collar, and includes a movable contact electrically connected with one terminal of a switch 101. A pair of iixed contacts 102 and 103 are mounted adjacent the movable contact 100, and the iixed contacts 102 and 103 are mounted close to the movable contact 100 to maintain a minimum gap.

When the pendulum 96 moves from a center position, the movable contact 100 engages one of the fixed contacts 102 or 103 and remains in engagement therewith until the pendulum completes its swing in one direction and then reverses toward the opposite direction. Thus, this arrangement causes the movable contact 100 to engage one of the fixed contacts 102 and 103 until the pendulum swings to a peak indicated as A and B, and when it reverses direction the movable contact 100 engages the other iixed contact 102 or 103. In this manner, each half period of a ship may be readily and simply sensed.

The fixed contact 102 is connected to a coil 105 of a double pole double throw relay 106, and the contact 103 is connected to a coil 107 of a similar relay 108. The other ends of the coils 105 and 107 are connected together and to a terminal of a voltage source 109, such as a battery. The other terminal of a voltage source 109 is connected to the remaining terminal of the switch 101. The relays 106 and 108 include respective movable contactors 110 and 111, and normally closed lower contacts and normally open upper contacts. Another voltage source 113 is connected through a switch 114 through the upper contacts of the relays 106 and 108 to respective cam operated limit switches 115 and 116, and to respective reset relays 117 and 118. The limit switch 115 normally provides a current path to a motor and an electromagnetic clutch 121, and the limit switch 116 provides a similar current path to a motor 122 and an electromagnetic clutch 123. The remaining terminals of the motors 120, 122 and clutches 121, 123 are connected to the other terminal of the voltage source 113 which also is connected to the remaining terminals of the reset relays 117 and 118.

Assuming that the switches 101 and 114 are closed and the pendulum -96 commences to swing counterclockwise toward its peak A, the movable contact 100 engages the fixed contact 102 thereby energizing the Winding 105 of the relay 106 causing the upper contacts thereof to be closed and the lower contacts thereof to be open. In this case the motor 120 and the clutch 121 are energized from the source 113 through the upper contacts of the relay 106 and the normally closed contacts of the limit switch 115. This causes a shaft 125 to be rotated thereby rotating cams 126 through 130. When the relay 106 is energized, the reset relay 117 also is energized through the upper contacts of the relay 106 to reset a shaft 131 coupled through the clutch 123 to the motor 122 to a zero position by means of a rack and pinion mechanism 132.

The shaft 125 continues to rotate until the pendulum 96 reaches point A and reverses direction and commences to swing clockwise toward peak B. When this occurs, the movable contact 100 swings into engagement with the fixed contact 103. During this transition of the movable contact 100 from the fixed Contact 102 to the xed contact 103 and while the contact 100 is not touching either contact 102 or 103, the position of the shaft 125 is sensed by means of one of the cam operated switches 133 through 136 operated by one of the respective cams 127 through to operate the control circuit 90 as will be discused in greater detail subsequently.

When the movable contact 100 engages the iixed contact 103, the winding 107 of the relay 108 becomes energized thereby closing the upper contacts and opening the lower contacts thereof. When the upper contacts of the relay 108 are closed, the motor 122 and clutch 123 are energized from the source 113 through the closed contacts of the limit switch 116. Also, the relay 118 is energized therebf resetting the shaft 125 to its initial or zero position through a rack and pinion mechanism 137. The motor 122 turns the shaft 131 through the clutch 123 until the lpendulum reaches the opposite peak B and reverses direction (to a counterclockwise rotation) thereby causing the movable contact 100 to move away from the fixed contact 103. Cams 138 through 142 are carried on the shaft 131, and these cams are like the respective cams 126 through 130. The set of these cams is illustrated in FIG- URE 6 and will be discussed subsequently. The cams 126 and 138 are merely limit cams to open the respective limit switches 115 and 116 in the event the respective shafts rotate close to three hundred and sixty degrees. The remaining cams operate respective cam operated switches as a function of the angular rotation of the respective shaft. Switches 144 through 147 are operated by the respective cams 139 through 142 in the same manner as switches 133 through 136 are operated by respective cams 127 through 130. It will be seen that each of the shafts 125 and 131 is rotated a period of time equal to a respective half period of the ship for each roll thereof.

A power source 149, such as a battery, is coupled through a switch 150 and the lower contacts of the relays 106 and 108 to one terminal of each of the cam operated switches 133 through 136 and 144 through 147. Lines 151 through 154 are connected to the other terminals of the respective switches 133 through 136 and 144 through 147. Depending upon which one of these cam operated switches is closed, a circuit will be completed to the control circuit 90 through one of the lines 151 through 154 and a common line 155 connected to the source 149. As will appear subsequently, the control circuit 90 provides an output signal to control the actuator 91 which in turn controls the natural period of the passive tank stabilizer.

The set of cams 126 through 130 is like the set of cams 138 through 142, and the former are shown in FIGURE 6. These cams 126 through 130 include respective lobes 160 through 164 which operate the respective switches engaging the cams. FIGURE 6 gives an example configuration for each of the cams illustrating the displacement from a zero position of the beginning of the lobes as well as the duration of the lobes. Two time gures are given in FIGURE 6, with the larger underlined number being the time in seconds (for a half period) provided by the cams for a full size .ship in which the cam shafts operate at 6.122 revolutions per minute, yand the smaller time numbers indicate the time in seconds (for a half period) used for cams for a scale of one-halt` inch equals one foot model test system having shafts operating at 30 revolutions per minute. A slight space is provided between the lobes of adjacent cams so two cam operated switches are not operated simultaneously.

It will be apparent through a consideration of FIG- URES a and 6 that if a half period of the pendulum 96 is 2.45 seconds or less, none of the lobes on any of the cams operate any of the respective switches, in which case the setting of the gate remains as set for a previous roll of the ship. If, for example, the period is just over 2.45 seconds, the lobe 161 on the cam 127 (or the cam 139 as the case may be) will close the cam operated switch 133 thereby completing a circuit from the source 149 to the line 151 at the instant the pendulum reaches its -peak and reverses direction causing the contact 100 to move away from the xed contact 102 and allowing the circuit to be completed through the lower contacts of the relays 106 and 108. This control signal applied across the line 151 and the common line 155 occurs only when the movable contact 100 is making its transition between the xed contacts, and this control signal is used to energize a latching relay in the control circuit 90 and also be deenergize any latching relay which previously was latched. It thus will be seen that depending upon the time of a half period of the pendulum 96, either none of the cams 127 through 130 (or cams 138 through 142) operates its respective switch7 or a particular one of these cams operates its respective switch. If no switch is operated, no control signal is applied to the control circuit .90 ia which case the gate of the passive tank stabilizer remains at the position set by the actuator when the previous measurement was made.

The lines 151 through 154 are connected to windings of respective latching relays 170 through 173 along with the common line 155. Each of these relays is alike, and the relay 170 for example includes an upper Winding 174 which serves to latch the relay and close the contacts thereof, and a lower winding 175 which serves to unlatch the relay and open the contacts. One contact of each of these relays is connected through a switch 177 to a voltage source 178, and the other contacts thereof are connected to respective output lines 179 through 182. The output lines also are connected to respective indicator lamps 183 through 186, `which in turn are connected to a common line 187 which is connected to the other terminal of the source 178. It will be apparent, that depending upon which of the lines 151 through 154 from the timer 89 is energized, one of the relays 170 through 173 will become energized thereby providing an output signal on one of the lines 179 through 182.

Inasmuch as the relays 170 through 173 are of the latching type, the lower banks of relays in the control circuit shown in FIGURE 5a are employed for resetting the latching relays whenever a different latching relay is energized from the timer 89. The lines 151 through 154 are connected to the lower banks of relays 190 through 201 along with the common line 155. Each of these relays has a contact on the right side connected through a line 203 and a switch 204 to a voltage source 205, the other terminal of which is connected to each of the lower windings in the relays through 173. The other terminals of the lower windings of the relays 170 through 173 are connected with the left hand contacts of the respective lower banks of relays. For example, the right hand contacts of the relays 190 through 192 are connected through the line 203 and the switch 204 to the lower terminal of the source 205. The upper terminal of the source 205 is connected to the winding of the latching relay 170, and in a similar manner to the lower windings in the latching relays 171 through 173. The remaining terminal of the winding 175 is connected to all of the left-hand contacts of the relays through 192. In a similar manner, the maining terminals of the lower windings of the latching relays 171 through 173 are connected to all of the left-hand contacts of the respective banks of relay associated therewith.

The line 151 is connected to the relays 193, 196 and 199. The line 152 is connected to the relays 190, 197 and 200. The line 153 is connected to the relays 191, 194, and 201. The line 154 is connected to the relays 192, 195 and 198, and the common line 155 is connected to all of the remaining terminals of the windings in the relays 190 through 201.

Assuming that the cam 127 in the timer 89 is rotated sufficiently to close the switch 133, the line 151 energizes the winding 174 of the latching relay 170 during the transition of the movable contact 100 `between the fixed contacts 102 and 103 thereby causing the contacts of the relay 170 to latch closed. This provides an output signal on the output line 179. At the time line 151 is energized, the relays 193, 196 and 199 also are energized and close their contacts thereby energizing the lower windings in the latching relays 171 through 173 to reset these latching relays. The output signal may be taken across the line 179 and the common line 187 and employed to operate the actuator which in turn moves the gate of the tank stabilizer to position 1." In a similar manner, other cam operated switches may cause other respective latching relays to provide an output signal.

A diagrammatic illustration of an exemplary actuator for a passive tank stabilizer is shown in FIGURE 5b. It will be appreciated that the actuator 91 may take any of various forms wherein conventional electrical, hydraulic or pneumatic power devices respond to any one of four control signals to move a gate 210 to any one of several positions. When no control signal has been applied to the actuator, the gate remains in a fully closed position. During operation of the control system .the gate remains set to a given position until a new control signal indicates that the position thereof should be changed. A single selectively operated motor may be employed, or a plurality of motors or rams may be used.

Four electrically operated rams 211 through 214 having different length strokes for example may be coupled with an actuator rod 215 which in turn is coupled through a rack 216 to the gate 210 to move the gate to any one of four positions. Lines 217 through 220 coupled with the respective rams 211 through 214 may be connected with the respective output control lines 179 through 182 of the control circuit 90. The remaining input lines to the rarns may be coupled with the common output control line 187. A switch 222 may be operated lby an arm 223 to control a green indicator light 224 and a red indicator light 225 located on the bridge of a ship. The lights 183 through 186 in the control system 90 (FIGURE 5a) are located on the bridge of the ship to indicate gate position. When the gate 210 is in any raised position, the green light 224 will be on, and when the gate is fully lowered to close the duct of the tank Stabilizer, the red light 225 will be energized.

When it is desired to shut down the tank stabilizer, e.g., completely lower the gate, it is preferable that the last portion of the duct such as between the vane 35 and the bottom 12 as shown in FIGURE 2a, be closed slowly over several periods, such as four to five rolls. In this manner, the period of the tank continually gets longer thereby enabling the liquid in the tank to end up level on each side thereof. In order to accomplish this, an electric motor 227 may be provided to drive the rack 216 to move the gate down through the last increment. The motor 227 is operated by a source 228 and a switch 229, and a limit switch 230 operated by an arm 231 may be provided to turn off the motor 227 when the tank is completely closed. Preferably, means are provided to disengage the motor 227 from the rack 216 except during the last incremental closure of the tank so as to reduce any loading on the actuator rod 215. A manual crank operating through gearing may be used to operate the gate 210 through its range, or to lower the gate to a closed position, in case of power failure.

While it is possible to compute the physical dimensions of the stabilizer parts for a given size ship, the actual dimensions and steps of operation may be determined by actual tests and adjustments of the parts. These adjustments may have to be varied in accordance with changes in loading of the ship.

While specific embodiments had been shown for purposes of illustration, various changes and modifications will be apparent to a person skilled in the art.

What is claimed is:

1. A method for stabilizing the roll of a vessel comprising confining a volume of liquid within said vessel, and allowing said liquid to move back and forth toward the sides of said vessel, measuring the period of a roll cycle of saidvessel, and controlling the natural period of movement of said volume of liquid as a function of each period of roll of said vessel, and continuously maintaining the period of said volume of said liquid in excess of each period of said vessel.

2. The method set forth in claim 1 in which the period of movement of said liquid is controlled as a function of successive portion-s of the period of roll of said vessel.

3. A tank stabilization system for stabilizing a vessel comprising tanks on opposite sides of the vessel, connecting passages interconnecting said tanks for the flow of liquid therebetween, said passages having substantially the same cross-sectional area throughout their extent whereby the liquid is unimpeded in its ow therethrough, means for varying the cross-sectional area of said pas sages through which liquid may flow between said tanks,

for thereby varying the natural period of flow of said liquid from one tank to the other, period sensing means for sensing the period of roll of said vessel, and control means responsive to said period sensing means connected to actuate said means for varying the cross-sectional area of said passages, in a sense to maintain the natural period of said liquid in excess of the period of said vessel.

4. A tank stabilizing system as set forth in claim 3 wherein said period sensing means comprises a stable element adapted to maintain a given plane regardless of the instantaneous position of the vessel, a pair of contact members mounted to move with the vessel and poSitioned to be closed selectively by said stable element when the vessel rolls in one direction or the other from its level position, a rotatable shaft and a driving motor therefor, means responsive to the closing of one of said contact members connected to cause said motor to drive said shaft continuously at a uniform rate while said contact member is closed, whereby the angular displacement of said shaft is a function of the period of time of said roll.

5. Apparatus as set forth in claim 4 wherein said control means includes a control `device for said stabilization system movable in a plurality of successive positions, and means responsive to the angular displacement of said shaft to vary the position of said control device.

6. A tank stabilization system as set forth in claim 3 wherein said connecting means comprises a duct having a plurality of vanes positioned therein and spaced to form separate connecting passages, and said tank stabilization means includes gate means selectively positionable to block one or more of said passages to change the natural period of said tank means, and said control means is connected to operate said gate means.

7. A tank stabilization system as set forth in claim 3 wherein said period sensing means senses fractions of each period of said vessel.

8. A tank stabilization system as set forth in claim 6 wherein said tank means includes sides, a bottom and means therein spaced from said bottom and defining said reservoirs and said duct, said vane means includes a plurality of plates mounted in said duct substantially parallel to said bottom, and said gate means is movable in a direction substantially perpendicular to said plates to selectively block respective passages for selectively changing the natural period of said tank means.

9. A tank stabilization system as set forth in claim 6 wherein said gate means includes a plurality of plate members for closing the ends of one or more of said passages.

10. A tank stabilization system as set forth in claim 6 wherein said plates mounted in said duct have openings therein, and said gate means includes a plate member movable into said openings in said plates to selectively block one or more of said passages.

11. A tank stabilization system as set: forth in claim 3 wherein said means to vary the cross-.sectional area of said passage includes a movable member having a surface defining one side of said passage, the position of said movable member being variable to change substantially the entire cross-sectional area of said duct.

12. A tank stabilizer as set forth in claim 6 in which the gate means is disposed at the ends of said passages.

13. A tank stabilizer as set forth in claim 6 in which said vanes have openings therein, and said gate means is movable into said openings in said vane means to selectively block one or more of said passages.

14. Apparatus as set forth in claim 4 in which said stable element comprises a pendulum.

15. Apparatus as set forth in claim 4 including reset means to reset said shaft during the reversal of the direction of roll.

16. Apparatus as set forth in claim 5 including relay means connected to lock said control device in a selected position.

References Cited UNITED STATES PATENTS 10/1911 Frahm 114-125X 12/ 1951 Hauptman 244- 80X 10/ 1958 Owen.

6/ 1960 Gallagher.

12 3,256,848 6/1966 Ripley 114-125 3,269,346 8/1966 Bell 114-125 FOREIGN PATENTS 684,210 11/1939 Germany.

TRYGVE M. BLIX, Primary Examiner.

U.S. C1. X.R. 144-122 

